<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    

<input type="text" id="n1">
<select id="opt">
    <option value="+">+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
</select>
<input type="text" id="n2">
<span onclick="count()">=</span>
<span id="res"></span>
<script>
    function count() {
        let n1 = document.getElementById("n1").value;
        let opt = document.getElementById("opt").value;
        let n2 = document.getElementById("n2").value;
        document.getElementById("res").innerHTML = eval(n1 + opt + n2);
    }
</script>

</body>
</html>